The idea that T. rex might have been a scavenger has been around since the early 1900s. The most famous proponent of the theory today is John R. Horner of Montana State University. According to Horner and other paleontologists, there are lots of reasons why T. rex may not have been a predator.

­One argument plays off the theory that dinosaurs evolved into birds. Some of the world's largest flying birds, such as condors, are scavengers -- they eat what they find instead of what they kill. By this logic, really big dinosaurs might have been scavengers, just like their really big avian counterparts today.

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This is mostly speculative, but some aspects of T. rex anatomy suggest that it was a scavenger. Its nasal passages, for instance, are huge, potentially perfect for smelling faraway carrion. A tyrannosaurus's teeth and jaw are made for biting -- hard. When a T. rex closed its mouth, the lower teeth met the inside of the upper teeth, concentrating lots of force upward from the inside and downward from the outside. This force could break a bone just like you could break a stick if you bend it with two hands. Paleontologists have also analyzed a coprolite, or a pile of fossilizedT. rex dung, and found bone fragments inside. This may mean that the dinosaur relied on picked-over bones for nourishment. To some, the presence of lots of broken teeth also suggests that T. rex chewed its way through bones out of necessity, damaging its teeth in the process.

The scavenger theory applies to a tyrannosaur's body, too. Calculations made by paleontologist James Farlow suggest that T. rex was so massive that it would have sustained life-threatening injuries if it fell while running [source: Hecht]. There's also the matter of T. rex's almost comically undersized forelimbs, which would not be of use in breaking a fall or helping the animal regain its footing.

Scientists also debate whether the dinosaur could run at all, and it would need to run to capture prey. There's no footprint, or trackway, evidence to suggest that it did, but most trackways aren't big enough to encompass the stride a T. rex would have used when running. Several studies analyzing different aspects of tyrannosaur physiology suggest that T. rex's speed topped out at about 10 meters per second, or 22 mph, but others suggest that it may have run much faster. Researchers John Hutchinson and Mariano Garcia suggest that tyrannosaur legs couldn't have supported enough muscle to allow fast running [source: Fitzgerald].

According to Graeme D. Ruxton and David C. Houston of the University of Glasgow, T. rex could have supported itself by eating only dead animals if the ecosystem had about the same amount of carrion as today's Serengeti. The two came to this conclusion by studying the energy it would take to find food compared to the energy the food would provide the dinosaur [source: Ruxton and Houston].

So it was possible that T. rex was a scavenger -- but was it likely? Next, we'll explore some of the most common counterarguments.